1. Field of the Invention
The present invention relates, broadly, to a nickel oxide-hydrogen secondary cell, and more particularly, to a method of manufacturing a hydrogen absorption alloy electrode containing a hydrogen absorption alloy as the main component of the negative electrode.
2. Description of the Related Art
It is known to the art that a so-called "hydrogen absorption alloy", i.e., an alloy capable of reversibly absorbing and desorbing hydrogen, acts not only on a gaseous hydrogen but also on the ionic hydrogen such as proton or hydronium ion so as to electrochemically absorb hydrogen atoms. The negative electrode of a secondary cell using such a hydrogen absorption alloy does not contain a harmful metal. In addition, the hydrogen absorption alloy is light in weight, compared with cadmium used in a nickel-cadmium secondary cell for forming the negative electrode or lead used in a lead secondary cell for forming the negative electrode, with the result that the hydrogen absorption alloy leads to a high energy density per unit weight. It follows that it is possible to prepare a secondary cell having a high energy density, if a negative electrode using a hydrogen absorption alloy is used in combination with a positive electrode formed of a nickel compounds including Ni(OH).sub.2 and NiOOH.
In a nickel oxide-hydrogen secondary cell having a positive electrode formed of nickel oxide and a negative electrode containing a hydrogen absorption alloy as a main component, reaction 1 given below takes place at the negative electrode. On the other hand, reaction 2 given below takes place at the positive electrode: ##STR1## "M" in formula (1) represents a hydrogen absorption alloy.
Several methods are known to the art with respect to the manufacture of a negative electrode for a secondary cell using a hydrogen absorption alloy, including, for example, a method in which a powdery hydrogen absorption alloy is shaped by using a suitable binder, a method in which a powdery hydrogen absorption alloy is sintered under an inert gas atmosphere, a hydrogen gas atmosphere or under vacuum, and a method in which a hydrogen absorption alloy is amorphitized. Among these known methods, the method involving the sintering treatment and the amorphiti for making the hydrogen absorption alloy amorphous require complicated techniques and are poor in shaping capability, making it practically difficult to incorporate the electrode formed by these methods in a cell to manufacture a satisfactory cell.
Under the circumstances, the method in which a powdery hydrogen absorption alloy is shaped by using a suitable binder is employed in general for the manufacture of the electrode. In this method, a fluorine resin binder is used most widely in the light of the bonding strength and workability.
A method of manufacturing a hydrogen absorption alloy electrode using a fluorine resin binder is disclosed in, for example, Published Unexamined Japanese Patent Application No. 61-66372, Published Unexamined Japanese Patent Application No. 61-124054 and U.S. Pat. No. 4,636,445. In the method disclosed in these prior arts, a thickening agent such as carboxymethyl cellulose (CMC) or polyvinyl alcohol (PVA) is added together with water to a mixture of the powdery hydrogen absorption alloy and the binder so as to uniformly knead the mixture, followed by coating, drying and rolling the kneaded mass so as to obtain a desired electrode. In this method, however, the hydrogen absorption alloy is oxidized by the water added in the kneading step or by the heat applied in the drying step, giving rise to serious problems. For example, the negative electrode capacity is lowered, and the negative electrode is lowered in its capability of reducing oxygen gas generated at the positive electrode during the over-charging operation, leading to a shortened life of the cell. It is also known to the art that a powdery hydrogen absorption alloy is uniformly mixed with a powdery binder, followed by forming an electrode by means of pressing. In this method, however, the electrode surface is excessively covered with the binder, leading to a low electrode capacity.
As described above, the hydrogen absorption alloy electrode manufactured in any of the conventional methods using a binder is low in a capability of reducing oxygen gas generated at the positive electrode during over-charging, or in an electrode capacity.